Modulated reflectance measurement system with multiple wavelengths

ABSTRACT

A modulated reflectance measurement system includes three monochromatic diode-based lasers. Each laser can operate as a probe beam or as a pump beam source. The laser outputs are redirected using a series of mirrors and beam splitters to reach an objective lens. The objective lens focuses the laser outputs on a sample. Reflected energy returns through objective and is redirected by a beam splitter to a detector. A lock-in amplifier converts the output of the detector to produce quadrature (Q) and in-phase (I) signals for analysis. A Processor uses the Q and/or I signals to analyze the sample. By changing the number of lasers used as pump or probe beam sources, the measurement system can be optimized to measure a range of different samples types.

PRIORITY CLAIM

This application is a continuation of U.S. Pat. No. 11/302,674, filedDec. 14, 2005 which is in turn a continuation of U.S. Pat. No.10/439,455, filed May 16, 2003. The present application claims priorityto U.S. Provisional Patent Application Ser. No. 60/390,487, filed Jun.21, 2002, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The subject invention relates generally to optical methods forinspecting and analyzing semiconductor wafers and other samples. Inparticular, the subject invention relates to methods for increasing theaccuracy and flexibility of systems that use modulated opticalreflectivity to analyze semiconductor wafers.

BACKGROUND OF THE INVENTION

There is a great need in the semiconductor industry for metrologyequipment that can provide high resolution, nondestructive evaluation ofproduct wafers as they pass through various fabrication stages. Inrecent years, a number of products have been developed for thenondestructive evaluation of semiconductor samples. One such product hasbeen successfully marketed by the assignee herein under the trademarkTherma-Probe. This device incorporates technology described in thefollowing U.S. Pat. Nos. 4,634,290; 4,646,088; 5,854,710; 5,074,669 and5,978,074. Each of these patents is incorporated in this document byreference.

In the basic device described in the patents, an intensity modulatedpump laser beam is focused on the surface of a sample for periodicallyexciting the sample. In the case of a semiconductor, thermal and plasmawaves are generated in the sample that spread out from the pump beamspot. These waves reflect and scatter off various features and interactwith various regions within the sample in a way that alters the flow ofheat and/or plasma from the pump beam spot.

The presence of the thermal and plasma waves has a direct effect on thereflectivity at the surface of the sample. Features and regions belowthe sample surface that alter the passage of the thermal and plasmawaves will therefore alter the optical reflective patterns at thesurface of the sample. By monitoring the changes in reflectivity of thesample at the surface, information about characteristics below thesurface can be investigated.

In the basic device, a second laser is provided for generating a probebeam of radiation. This probe beam is focused colinearly with the pumpbeam and reflects off the sample. A photodetector is provided formonitoring the power of reflected probe beam. The photodetectorgenerates an output signal that is proportional to the reflected powerof the probe beam and is therefore indicative of the varying opticalreflectivity of the sample surface.

The output signal from the photodetector is filtered to isolate thechanges that are synchronous with the pump beam modulation frequency. Inthe preferred embodiment, a lock-in detector is used to monitor themagnitude and phase of the periodic reflectivity signal. This outputsignal is conventionally referred to as the modulated opticalreflectivity (MOR) of the sample.

In the early commercial embodiments of the Therma-Probe device, the pumpand probe laser beams were generated by gas discharge lasers.Specifically, an argon-ion laser emitting a wavelength of 488 nm wasused as the pump source. A helium neon laser operating at 633 nm wasused as the probe source. More recently, solid-state laser diodes havebeen used and are generally more reliable and have a longer lifetimethan the gas discharge lasers. In the current commercial embodiment, thepump laser operates at 780 nm while the probe laser operates at 670 nm.

In practice, the response of the sample to the pump beam is dependent tosome degree on the wavelength. Further, the sensitivity of the system isalso dependent on either pump or probe beam wavelength and therelationship between the pump and probe beam wavelengths. Thecombination of wavelengths selected by the assignee in its commercialembodiment is intended to strike a balance allowing measurements over arelatively broad range of samples. However, it can be shown that certainsamples could be more accurately measured if the pump and probe beamwavelengths were optimized for that sample type or sample range.

In the most common commercial application of the Therma-Probe, thedensity or dosage levels of implants in silicon are measured. While thecurrent pump and probe beam wavelengths provide good sensitivity acrossa relatively wide range of doses, certain regions are less sensitivethan others. Accordingly, it would be a benefit if the user waspermitted to select a particular set of wavelengths to perform certainmeasurements.

SUMMARY

The present invention provides a modulated reflectance measurementsystem with multi-wavelength measurement capability. For oneimplementation, the measurement system includes three monochromaticdiode-based or diode-pumped semiconductor lasers. Each laser can operateas a probe beam source or as a pump beam source. The laser outputs areredirected using a series of mirrors and beam splitters to reach anobjective lens. The objective lens focuses the laser outputs on asample. Reflected energy returns through objective and is redirected bya beam splitter to a detector. A filter prepares the outputs of thedetector for analysis by a processor. Typically, the filter includes alock-in amplifier that converts the output of the detector to producequadrature (Q) and in-phase (I) signals for analysis. The processortypically converts the Q and I signals to amplitude and/or phase valuesto analyze the sample. In other cases, the Q and I signals are useddirectly.

The use of three different lasers provides six possible combinationswhere a single probe beam is used with a single pump beam. Alternately,two lasers can be used to produce different probe beams while the thirdlaser produces the pump beam. In another variation, two lasers canproduce pump beams (at different modulation frequencies) while the thirdproduces a probe beam. Another configuration uses all three lasers toproduce intensity modulated pump beams. The light reflected by thesample originating from the first laser is monitored at the differencebetween the modulation frequencies of the second and third lasers. Thereflected light of the second and third lasers is monitored in ananalogous fashion. In this way, the present invention provides adynamically reconfigurable measurement system that can be optimized tomeasure a range of different sample types.

For another implementation, the measurement system includes a pump laserand a probe laser. One or both of these lasers are wavelength tunable.The pump laser and probe lasers are controlled by a modulator. The laseroutputs are redirected using a series of mirrors and beam splitters toreach an objective lens. The objective lens focuses the laser outputs ona sample. Reflected energy returns through objective and is redirectedby a beam splitter to a detector. A filter prepares the outputs of thedetector for analysis by a processor. Typically, the filter includes alock-in amplifier that converts the output of the detector to producequadrature (Q) and in-phase (I) signals for analysis. The processortypically converts the Q and I signals to amplitude and/or phase valuesto analyze the sample. In other cases, the Q and I signals are useddirectly. By selectively controlling the wavelengths produced by thepump laser and/or probe laser, the operation of modulated reflectancemeasurement system can be optimized to measure a range of differentsample types.

For another implementation, of the measurement system pump and probelasers are added as modular subsystems. Typically, this includesseparate low-dose, mid-dose, high-dose, and all-dose modules. Each ofthese modules includes a pump laser and a probe laser having wavelengthsthat are selected to optimally analyze a particular range ofimplantation dosages. The all-dose module is intended to provide awideband tool that operates over a range of dosage levels. The low-dosemodule, mid-dose module, and high-dose module provide insight intodiscrete portions of that range. The modules share a set of commoncomponents, which typically include optics, a detector and a processor.By selectively enabling or disabling the modules (alone or incombination), the operation of the operation of modulated reflectancemeasurement system can be optimized to measure a range of differentsample types.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a modulated reflectance measurement systemthat uses two wavelength tunable laser sources.

FIG. 2 is a block diagram of a modulated reflectance measurement systemthat uses three single wavelength laser sources.

FIG. 3 is a schematic diagram of a modulated reflectance measurementsystem that uses four modular laser sources.

FIG. 4 is a block diagram of an implementation of the modulatedreflectance measurement system of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a modulated reflectance measurementsystem with multi-wavelength measurement capability. In FIG. 1, onepossible implementation for this system is shown and generallydesignated 100. As shown in FIG. 1, modulated reflectance measurementsystem 100 includes a pump laser 102 and a probe laser 104. At leastone, and for some implementations both, of these lasers are wavelengthtunable. Preferably, the output can be tuned over a range of at least 50nm. Pump laser 102 and probe laser 104 are controlled by a processor106. The time varying characteristics of the output of pump laser 102are controlled by a modulator 108.

The output of pump laser 102 and probe laser 104 are redirected by amirror 110 and a beam splitter 112, respectively. After beingredirected, the two outputs pass through an objective lens 114 and arefocused on a sample 116. The reflected energy returns through theobjective 114 and is redirected by a beam splitter 118 towards adetector 120. Detector 120 measures the energy reflected by sample 116and forwards a corresponding signal to a filter 122. Filter 122 includesa lock-in amplifier that uses the output of detector, along with theoutput of modulator 108 to produce quadrature (Q) and in-phase (I)signals for analysis. Processor 106 typically converts the Q and Isignals to amplitude and/or phase values to analyze the sample. In othercases, the Q and I signals are used directly. By selectively controllingthe wavelengths produced by pump laser 102 and/or probe laser 104, theoperation of modulated reflectance measurement system 100 may beoptimized to match the characteristics of sample 116.

In FIG. 2, a second possible implementation for the modulatedreflectance measurement system is shown and generally designated 200. InFIG. 2, modulated reflectance measurement system 200 includes threelasers 202 a through 202 c. Each laser 202 is typically monochromaticand each laser 202 typically operates at a different spectrum. Lasers202 are generally diode-based or diode-pumped semiconductor lasers.Solid-state laser diodes are available that have outputs throughout theentire visible spectrum as well as in the infrared and near UV. Lasers202 are controlled by a processor 204 and a modulator 206. Each laser202 is controlled independently allowing processor 204 to enable ordisable any of lasers 204. Lasers 204 that are enabled may be configuredto produce intensity-modulated outputs or configured to producenon-modulated (i.e., constant intensity) outputs.

The output of lasers 202 a, 202 b, and 202 c are redirected by a mirror208, a beam splitter 210 and a beam splitter 212, respectively. Afterbeing redirected, the outputs pass through an objective lens 214 and arefocused on a sample 216. The reflected energy returns through theobjective 214 and is redirected by a beam splitter 218 towards adetector 220. Detector 220 measures the energy reflected by sample 216and forwards a corresponding signal to a filter 222. Filter 222typically includes a lock-in amplifier that uses the output of detector,along with the output of modulator 206 to produce quadrature (Q) andin-phase (I) signals for analysis. Processor 204 typically converts theQ and I signals to amplitude and/or phase values to analyze the sample.In other cases, the Q and I signals are used directly.

A number of different configurations are available to control theoperation of lasers 202. These configurations include a single pumpsingle probe configuration where pump and probe beams are generated byrespective lasers 202. A multiple probe, single pump configuration isalso supported where two lasers 202 generate probe beams with theremaining laser 202 generating a pump beam. Similarly, a multiple pump,single probe configuration is supported where two lasers 202 generatepump beams with the remaining laser 202 generating a probe beam. Amultiple pump, multiple probe configuration is also available where allthree lasers 202 generate intensity-modulated beams.

For the single pump single probe configuration, processor 204 choosesone or more of lasers 202 to produce a pump beam and one or more oflasers 202 to produce a probe beam. The lasers selected to produce thepump beam are controlled by modulator 206 to have a time varying output.In the simplest case, one of lasers 202 provides the pump beam while oneof lasers 202 provides the probe beam. Since each laser 202 is typicallyconfigured to operate at a different wavelength, there are six possiblecombinations of different pump and probe beams. This allows modulatedreflectance measurement system 200 to be configured to analyze a rangeof samples having different characteristics.

For the multiple probe, single pump configuration, modulated reflectancemeasurement system 200 is configured to include two lasers 202 thatprovide the probe beam and one laser 202 provides the pump beam. Sincereflectance may be wavelength dependent, probing at multiple wavelengthscan be used to enhance the information obtained for some sample types.Since lasers 202 are fully interchangeable between pump and probe dutiesthere are three different configurations that include two probe lasers202. This allows the multiple probe, single pump configuration toanalyze a range of different samples types.

For the multiple pump, single probe configuration, two lasers 202provide the pump beam while the remaining laser 202 provides the probebeam. Since different wavelengths of light produce different thermal andplasma effects within sample 216, pumping at multiple wavelengths can beused to enhance the information obtained for some sample types. Sincelasers 202 are fully interchangeable between pump and probe duties thereare three different configurations that include two pump lasers 202.This allows the multiple pump configuration, single probe to analyze arange of different samples types.

When the multiple pump, single probe configuration is used, the lasers202 selected to produce the pump beam are controlled by modulator 206 tohave a time varying output. In such cases, different modulations willtypically be used to produce the different pump beams. It is alsopossible to measure (either alternatively or in addition) the reflectedlight of either of the two pump beams. Based on the optical heterodynetechnique as discussed in U.S. Pat. No. 5,206,710, either pump beamcould monitored as a probe beam.

For the multiple pump, multiple probe configuration, all three lasers202 generate intensity modulated beams, each at a different modulationfrequency. The light reflected by the sample originating from laser 202a is monitored at the difference between the frequencies with respect toboth lasers 202 b and 202 c. Light reflected by the sample originatingfrom laser 202 b is monitored at the difference between the frequencieswith respect to lasers 202 a and 202 c. Similarly, light reflected bythe sample originating from laser 202 c is monitored at the differencefrequencies with respect to lasers 202 b and 202 a. Thus, three lasers202 gives the possibility of simultaneous measurement at three differentprobe beam wavelengths, each at two different pump beam wavelengths.

The different configurations provide a flexible mechanism for optimizingmodulated reflectance measurement system 200 to analyze a range ofdifferent samples types. This is particularly true where a range ofdifferent density or dosage levels must be measured for differentsemiconductor wafers. It is also beneficial when analyzing ultra shallowjunctions and other semiconductor features.

In general, it should be appreciated that the particular combination ofcomponents shown in FIG. 2 is intended to be representative in nature—awide range of alternative configurations are possible. For example, asshown in FIG. 2 the optical path in modulated reflectance measurementsystem 200 is largely defined by a series of beam splitters 210, 212,218 and a mirror 208. The selection of particular splitter or mirrors isgoverned by the wavelengths. With appropriate coatings, the beamsplitters 210, 212, 218 can be fixed in position. However, it is alsopossible to move the mirrors into and out of the beam paths to controlthe propagation direction of the light. It is also possible to useshutters to control light propagation. It should also be appreciatedthat the use of three lasers 202 is only an example. The same techniquescan be extended to support any number of lasers.

In another embodiment, only two lasers 202 a and 202 b, each having adifferent wavelength output, are provided. The output of either or boththe laser can be intensity modulated. The user can select which of thetwo beams are monitored. In one configuration, laser 202 a acts as thepump and laser 202 b acts as the probe. In another configuration, laser202 a acts as the probe and laser 202 b acts as the pump. Both laserscan be modulated and either could act as the probe by monitoring thedifferent frequency. The user can select the appropriate configurationbased on the type of sample being measured. In order to obtain areasonable amount of additional information, the wavelength separationbetween the two lasers should be at least 50 nm and preferably 100 nm ormore.

In FIG. 3, a third possible implementation for the modulated reflectancemeasurement system is shown and generally designated 300. For thisimplementation, pump and probe lasers are added as modular subsystems.FIG. 3 shows four of these modular subsystems. In order, they are: alow-dose module 302, a mid-dose module 304, a high-dose module 306, andan all-dose module 308. Each of these modules includes a pump laser anda probe laser having wavelengths that are selected to optimally analyzea particular range of implantation dosages. The all-dose module 308 isintended to provide a wideband tool that operates over a range of dosagelevels. Low-dose module 302, mid-dose module 304, and high-dose module306 provide insight into discrete portions of that range. Each of thesemodules uses a set of common components (e.g.: optics, filter,processor) designated 310 in FIG. 3.

For the example of FIG. 3, the different modules are intended to analyzedifferent implantation dosage ranges. It should be appreciated that thisis a representative implementation. Modules could also be selected toanalyze other features, such as a range of modules designed to analyzedifferent ultra shallow junctions.

FIG. 4 shows a modulated reflectance measurement system 400 implementedusing the modular approach. System 400 includes a low-dose module 402, amid-dose module 404, a high-dose module 406, and an all-dose module 408(or, equivalently different modules for different ultra shallowjunctions). An X-Y stage 410 that allows relative positioning for asample is shared by each of the modules. The optical path withinmodulated reflectance measurement system 400 is defined by mirrors 412a, 412 b and beam splitters 414 a and 414 b. A rotating mirror 416selects the module (or modules) that have access to X-Y stage 410 andsample at any given time. A series of shutters 418 a through 418 dcontrols optical propagation within modulated reflectance measurementsystem 400. For example, if high-dose 406 module is used, shutters 418 athrough 418 c block the beams from low-dose module 402, a mid-dosemodule 404, and all-dose module 408.

The separate modules shown in FIGS. 3 and 4 may also be used in parallelto produce multiple pump or multiple probe beams. To support the use ofmultiple probe beams, beam splitters 414 a and 414 b are implementedusing a suitable dichroic design. As discussed previously, reflectancemay be wavelength dependent. As a result probing at multiple wavelengthscan be used to enhance the information obtained for some sample types.Multiple pump beams are typically accommodated using differentmodulation frequencies. Since different wavelengths of light producedifferent thermal and plasma effects, pumping at multiple wavelengthscan be used to enhance the information obtained for some sample types.The modular approach provides a flexible mechanism for optimizingmodulated reflectance measurement system 300/400 to analyze a range ofdifferent samples types. This is particularly true where a range ofdifferent density or dosage levels must be measured for differentsemiconductor wafers. It is also beneficial when analyzing ultra shallowjunctions and other semiconductor features.

1. An apparatus for evaluating a semiconductor sample comprising: first,second and third lasers each for generating an output beam at adifferent wavelength; optical elements for focusing the output beamscollinearly onto the sample; a modulator coupled to said first andsecond lasers; a detector for monitoring the reflected portion of anon-modulated beam and generating output signals in response theretothat correspond to the modulated optical reflectivity of the sample; afilter for filtering the output signals; and a processor for controllingthe modulator to selectively modulate one of the first and second lasersto operate as a pump beam to generate thermal and plasma waves in thesemiconductor sample which modulate the optical reflectivity of thesample and wherein either the other of the first and second lasers orthe third laser functions as a non-modulated probe beam and wherein theprocessor functions to evaluate the sample based on the filtered outputsignals.
 2. An apparatus as recited in claim 1, wherein said first lasergenerates UV radiation.
 3. An apparatus as recited in claim 1, whereinthe processor evaluates the implantation dose of the sample.
 4. Anapparatus as recited in claim 1, wherein the processor evaluates anultrashallow junction in the sample.
 5. An apparatus for evaluating asemiconductor sample comprising: first, second and third lasers each forgenerating an output beam at a different wavelength; optical elementsfor focusing the output beams colinearly onto the sample; a modulatorcoupled to said first and second lasers; a detector for monitoring thereflected portion of a non-modulated beam and generating output signalsin response thereto that correspond to the modulated opticalreflectivity of the sample; a filter for filtering the output signals;and a processor for controlling the modulator to selectively modulateone of the first and second lasers to operate as a pump beam to generatethermal and plasma waves in the semiconductor sample which modulate theoptical reflectivity of the sample and wherein the third lasersfunctions as a non-modulated probe beam and wherein the processorfunctions to evaluate the sample based on the filtered output signals.6. An apparatus as recited in claim 5, wherein said first lasergenerates UV radiation.
 7. An apparatus as recited in claim 5, whereinthe processor evaluates the implantation dose of the sample.
 8. Anapparatus as recited in claim 5, wherein the processor evaluates anultrashallow junction in the sample.
 9. An apparatus for evaluating asemiconductor sample comprising: first, second and third lasers each forgenerating an output beam at a different wavelength; optical elementsfor combining the output beams and for focusing the beams onto a spot onthe sample; a modulator coupled to said first and second lasers; adetector for monitoring the reflected portion of a non-modulated beamand generating output signals in response thereto that correspond to themodulated optical reflectivity of the sample; a lock-in amplifier forfiltering the output signals; and a processor for controlling themodulator to selectively modulate one of the first and second lasers tooperate as a pump beam to generate thermal and plasma waves in thesemiconductor sample which modulate the optical reflectivity of thesample and wherein either the other of the first and second lasers orthe third lasers functions as a non-modulated probe beam and wherein theprocessor functions to evaluate the sample based on the filtered outputsignals.
 10. An apparatus as recited in claim 9, wherein said firstlaser generates UV radiation.
 11. An apparatus as recited in claim 9,wherein said third laser functions as said non-modulated probe beam. 12.An apparatus as recited in claim 9, wherein the processor evaluates theimplantation dose of the sample.
 13. An apparatus as recited in claim 9,wherein the processor evaluates an ultrashallow junction in the sample.14. A method of evaluating a semiconductor sample by monitoring changesin the modulated optical reflectivity of the sample induced by periodicexcitation by a modulated pump beam, said method using a device havingfirst, second and third lasers each for generating an output beam at adifferent wavelength, said device including optical elements forcombining the output beams and for focusing the beams onto a spot on thesample, said method comprising the steps of: selecting one of saidlasers to be a pump beam; intensity modulating said one laser in amanner to generate thermal and plasma waves in the semiconductor samplewhich modulate the optical reflectivity of the sample; selecting one ofthe two remaining unselected lasers to be a probe beam; monitoring thereflected portion of probe beam and generating output signals inresponse thereto that correspond to the modulated optical reflectivityof the sample; filtering the output signals; and evaluating the samplebased on the filtered output signals and wherein the selections of theparticular lasers as a pump beam and as the probe beam are made tooptimize performance based on the particular sample and thecharacteristics to be evaluated.
 15. A method as recited in claim 14,wherein one of the selected lasers generates UV radiation.
 16. A methodas recited in claim 14, wherein the probe beam is not modulated.
 17. Amethod as recited in claim 14, the implantation dose of the sample isevaluated.
 18. A method as recited in claim 14, wherein an ultrashallowjunction in the sample is evaluated.